1864.] T. STERRY HUNT ON LITHOLOGY. 25 



this, there is a gradual passage through granular into compact 

 varieties of rock. Most of these are simply finely granular, and 

 are rightly entitled to the distinction of crypto-crystalline ; but 

 others, like the pitchstones, obsidians, and lavas, are apparently 

 amorphous, and are natural glasses. In some cases the constituent 

 minerals may be so arranged as to give a schistose or a gneissoid 

 form to a rock. This arrangement is generally to be looked upon as 

 an evidence of stratification ; but something similar is occasionally 

 observed in eruptive masses. In the latter case it generally 

 seems to arise from the arrangement of crystals during the 

 movement of the half-liquid crystalline mass ; but it may in some 

 instances arise from the subsequent formation of crystals arranged 

 in parallel planes. 



See on this point Naumann On the Probahle Eruptive Origin of 

 Several Kinds of Gneiss, etc. ; Leonhard and Bronn, Neues Jahr- 

 buch for 1847, and Poulett Scrope, Geo!. Journal, xii, 345. I 

 consider however that their views are to be adopted with great re- 

 serve, and admitted only in a very few cases. The ribbanded struc- 

 ture of some porphyries and clinkstones, as noticed by Scrope, is 

 undoubtedly the result of movements in the liquid mass, and the 

 same is true of some of the ofranitoid dolerites to be described in 

 the third part of this paper ; but the eruptive origin assumed by 

 Darwin, Naumann, and some others for great areas of gneiss and 

 gneissoid granite, seems to a student of the crystalline rocks of this 

 continent utterly untenable. As has been already remarked, the 

 progress of each year's investigation restores to the category of 

 indigenous rocks many of those previously regarded as eruptive, 

 and will, I am convinced, csnfirm the principle which I have laid 

 down of the comparative rarity of exotic rocks in crystalline 

 and in metamorphic regions. 



Occasionally the crystallization of a rock takes places around cer- 

 tain centres, givino^ rise to rounded masses which have a radiated or 

 a concentric structure, and constitute the so-called globular or orbi- 

 cular ro-ks. Distinct crystals of some mineral, generally feld- 

 spar, augite, or olivine, are often found imbedded in rocks having a 

 compact base. To such rocks the name of porphyry is given, and 

 by analogy a rock with agranular base enclosinir distinct crystals 

 is designated as porphyritic or porphyroid. Amorphous or vitreous 

 rocks, as pitchstones, are in like manner sometimes porphyritic- 

 The name of porphyry, at first given to a peculiar type of feld- 



