i6 



NA TURE 



[Nov. 2, 1 87 1 



distinctive characters of the three groups of crystalline strata just 

 mentioned, which will be shown in the sequel to have an impor- 

 tance in geology beyond the limits of the Appalachians. 



I. T/ii! AJirondacl; or Laiircntidc Sejics — The rocks of this 

 series, to which the name of the Laurentian system has been 

 given, may be described as chiefly firm granitic gneisses, often 

 very coarse-grained, and generally reddish or grayi.-h in colour. 

 Ttiey aie frequently hornblendic, but seldom or never contain 

 much mica, and the mica-schist (often accompanied with stau- 

 roli c, garnet, andalusite, and cyanite), so often characteristic of 

 the White Mountain series, are wanting among the Laurentian 

 ro.ks. They are also dest tute of argillites, which are found in 

 the other two series. Tt.e quarlzites, and the pyroxenic and 

 hornblendic rocks, associated with great formations of cryitalline 

 limestone, with graphite, and immense beds of magnetic iron 

 ore, give a peculiar character to portions of the Laurentian 

 system. 



2 The Green Moutttain St-rifs, — The quartzo-feldspathic rocks 

 of this series are to a considerable extent represented by a fine- 

 grained petrosilex or eurite, though they often assume the form 

 of a true gneiss, wliich is ordinarily more micaceous than the 

 typical Laurentian gniiss. The coarsegrained, porphyrilic, 

 reddish varieties common to the latter are wanting to the Green 

 Mountains, where the gneiss is generally of pale greenish and 

 graxith hues. Massive stratified dioiites, and epidotic and 

 chloritic rocks, often more or less schistose, with steatite, dark- 

 coloured serpentines and ferriferous dolomites and magnesites, 

 also characterise this gneissic series, and are intimately associated 

 with beds of iron ore, generally a slaty hematite, but occasionally 

 magnetite. Chrome, titanium, nickel, copper, antimony, and 

 gold are frequently met with in this series. The gneisses often 

 pasi into schistose micaceous qunrtzites, and the argillites, which 

 abound, frequently assume a soft, unctuous character, which has 

 acquired for them the name of talcose or nacreous slates, though 

 analysis shows them not to be magnesian, but to consist essentially 

 of a hydrous micaceous mineial. They are sometimes black and 

 graphitic. 



3. J'/ie U'/iile Mountain Scries. — This series is characterised 

 by the predominance of well-defined mica-schists, interstratified 

 with micaceous gneisses. These latter are ordinarily light- 

 coloured from the presence of white feldspar, and though gene- 

 rally fine in tcx'ure, are sometimes coarse-grained and por- 

 phyritic. They .are less strong and coherent than the gneisses of 

 the Laurentian, and pass, through the predominance of mica, 

 into mica-scliists, \\ hich are themselves more or less tender and 

 triable, and present every variety, from a coarse gneiss-like 

 aggregate down to a fine-grained schist, which passrs into 

 argilhte. The micaceous schists of this series ai"e generally much 

 richer in mica than those of the preceding series, and otten con- 

 tain a large proportion of well-defined crystalline tables belong- 

 ing to the species muscovite. The cleavage of these micaceous 

 schists is generally, if not always, coincident with the bedding, 

 but the plates of mica in the coarser-grained varieties are often 

 arranged at various angles to the cleavage and bedding-plane, 

 showing that they were developed after sedimentation, by crystal- 

 lisation in the mass, a circumstance which distirguislies them 

 from rocks derived from the ruins of these, which are met with 

 in more recent series. The White Mountain rocks also include 

 beds of micaceous quartzite. The basic silicates in this series 

 are represented chiefly by dark-coloured gnei.'ses and schists, 

 in which hornblende takes the place of mica. These pass 

 occasionally into beds of dark hornblende-rock, sometimes hold- 

 ing garnets. Beds of crystalline limestone occasionally occur in 

 the schists of the White Mountain series, and are sometimes ac- 

 companied by pyroxene, garnet, idocra'C, s[ bene, and graphite, 

 as in the corresponding rocks of the Laurentian, which this 

 series, in its more gneissic portions, closely resembles, though 

 apparently distinct geognostically. The limestones are intimately 

 associated with the highly micaceous schists, containing staurolite, 

 andalusite, cyanite, and garnet. These schists are sometimes 

 highly plumbaginous, as seen in the graphitic mica-schist holding 

 garnets in Nehon, New Hampshire, and that associated wiih 

 cyanite in Cornwall, Conn. To this third series of crystalline 

 schists belong the concretionary granitic veins atounding in 

 beryl, tourmaline, and lepidolite, and occasionally ccntaining 

 tinstone and columbite. Granitic veins in the Laurentian gneisses 

 frequently contain tourmaline, but have not, so far as is yet 

 known, yielded the other mineral species just mentioned.* 



IL The Origin ok Crystalline Rocks. — We now ap- 



Hunt, Note 



Jc 



proach the second part of our subject, namely, the genesis of 

 the crystalline schists. The origin of the mineral silicates, 

 which make up a great portion of the crystalline rocks 

 of ihe earth's surface, is a question of much geological 

 interest, wiiich has been to a great degree overlooked. The 

 gneisses, mica-schists, and argillites, of various geological periods 

 do not differ very greatly in chemical constitution from modern 

 mechanical sediments, and are now very generally regarded as 

 resulting from a molecular re arrangement of similar sediments 

 formed in earlier times by the disintegration of previously exis ing 

 rocks not very unlike them in composition ; the oldest knoAn 

 formations being still composed of crystalline stratified deposits 

 presumed to be of sedimentary origin. Before these the imagina- 

 tion conceives yet earlier rocks, until we reach the surface of un- 

 .stratified material, which the globe may be supposed to have 

 presen;ed before water had begun its work. It is not, however, 

 my present plan to consider this far off beginning of sedimentary 

 rocks, which I have elsewhere discussed. * 



Apart from the clay and sand-rocks just referred to, whose 

 composition may be said to be essentially quartz and aluminous 

 silicates, chiefly in the forms of feldspars and micas, or the re- 

 sults of their partial decomposition and disintegration, there is 

 another class of crystalline silicated rocks which, though far le^s 

 important in bulk than the last, is of great and varied interest to 

 the lithologist, the mineralogist, the geologist, and the chemist. 

 The rocks of this second class may be defined as consisting in 

 great part of the silicatesof theprotoxyd bases, lime, magnesia, and 

 ferrous oxyd, either alone, or in combination with silicates of 

 alumina and alkalies. They include the following as their chief 

 constituent mineral species : — Pyroxene, hornblende, olivine, ser- 

 pentime, talc, chlorite, epidote, garnet, and triclinic feldspars, 

 such as labradorite. The great types of this second class are not 

 less well defined than the first, and consist of pyroxenic and 

 hornblendic rocks, passing into diorites, diabases, ophiolites 

 and talcose, chloritic and epidotic rocks. Intermediate varieties 

 resulting from the association of the minerals of this class with 

 those of the first, and also with the materials of non-silicated 

 rocks, such as limestones and dolomites, show an occasional 

 blending of the conditions under which these various types of 

 rocks were formed. 



The distinctions just drawn between the two great divisions of 

 silicated rocks are not confined to stratified deposits, but are 

 equally well mai'ked in eruptive and unstratified masses, among 

 which the first type is represented by trachytes and granites, and 

 the second by dolerites and diori'es. This fundamental difTci- 

 ence between acid and basic rocks, as the two classes are called, 

 finds its expression in the theories of Phillips, Durocher, and 

 Bunsen, who have deduced all silicated rocks from two supposed 

 layers of molten matter within the earth's crust, consisting re- 

 spectively of acid and basic mi.xtures ; the trachytic and pyrox- nic 

 magmas of Bunsen. From these, by a process of partial crys- 

 tallisation and eliquation, or by commingling in various propor- 

 tions, those eruptive rocks which depart more or less from the 

 normal types are supposed by the theorists of this school to lie 

 generated. t The doctrine that these eruptive rocks are not de- 

 rived directly from a hitherto uncongealed nucleus, but are sofiened 

 and crystallised sediments, in fact that the whole of the rocks at 

 present known to us have at one time been aqueous deposits, has, 

 however, found its advocates. In support of this view, I have 

 endeavoured to show that the natural result of forces constantly 

 in operation tends to resolve the various igneous rocks mto two 

 classes of sediments, in which the two types are, to a great extent, 

 preserved. The mechanical and chemical agencies which trans- 

 form the crystalline rocks into sediments, separate these more or 

 less completely into coarse, sandy, permeable beds on the one 

 hand, and fine clayey impervious muds on the other. The action 

 of infiltrating atmospheric waters on the first and more silicious 

 sti-ata, removes from them lime, magnesia, iron-oxyd, and soda, 

 leaving behind silica, alumina, and potash — the elements of 

 gi'snitic, gneissic, and trachytic rocks. The finer and more alu- 

 minous sediments, including the ruins of the soft and easily 

 abraded silicates of the pyro.xene group, resisting the penetration 

 of the water, will, on the contrary, retain their alkalies, lime, 

 magnesia, and iron, and thus will have the composition of the 

 more basic rocks. J 



A little consideration will, however, show that this process, al- 

 though doubtless a true cause of diffei'ences in the composition of 



♦ Amer. Jour. Sci., IF. 1. 25. 



>t Hunt on Some Points of Chemical Geology, Qiiar. Jour.GeoLSoc, XV. 489. 



J l^u.ir. Jour. Geo]. Soc, XV. 489 ; also, Amer. Jour. Sci., II. xxx. 13V 



