HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 551 



3. The Argillaceous schists. Clay slates. 



Clay slate, or roofing slate, differs from the argillaceous rocks already 

 described (p. 537), in that it bas undergone a certain amount of dynamic 

 iuetamorphism which has developed in it its fissile cbaracter, and at tbe 

 same time more or less obliterated its Iragmental nature and converted 

 it in many cases into an extremely fine-grained mica schist. Tbe slates 

 as a whole may perhaps be considered as standing intermediate between 

 the true fragmental rocks and the crystalline schists, though a constant 

 gradation is readily traced from the unaltered argillites through the 

 cleavable slates to the so-called argillitic mica schists. The nature and 

 structure of the rocks here included may be best understood by refer- 

 ence to Professor Geikie's text book, pp. 125 and 228. A more instruct- 

 ive series than is here shown is displayed in the exhibits devoted to 

 dynamic geology. 



IV. — Bocks formed through igneous agencies. Eruptive.* 



This group includes all those rocks which having once been in a state 

 of igneous fusion owe their present structural and other peculiarities to 

 (1) The chemical composition of their original molten magmas, and (2) 

 the conditions under which these magmas cooled. As a matter of gen- 

 eral principle it may be stated that the greater the pressure under which 

 a rock solidifies and the slower and more gradual the cooling the more 

 perfect will be the crystalline structure. Hence it follows that the 

 older and deep-seated rocks which were forced up in the form of dikes, 

 bosses, or intrustive sheets .nto the overlying masses, and which have 

 become exposed only through erosion, are the more highly crystalline, 

 while those which like the modern lavas have flowed out upon the sur- 

 face are more or less felsitic or glassy. (See Figs. 1, 2, 3, and 4, PI. cxx. 

 The rocks from which these sections were prepared are of essentially 

 the same composition, the variations in structure being due to condi- 

 tions of cooling.) Intermediate structures have been produced through 

 a beginning of crystallization at certain depths below the surface, after 

 which and while a portion of the magma was still fluid it was pushed 

 upwards towards* the surface where cooling progressed more rapidly, 

 the result being a glassy or felsitic rock with scattering or porphyritic 

 crystals, as shown in the structural series. It is customary to speak of 

 this noncrystalline or unindividualized material as the groundmass or 

 base. Eocks which are crystalline throughout are said to be holocrys- 

 talline ; those which are without crystalline development, but consist 



"Advantage bas beeu taken of the opportunity here offered for bringing together 

 as large a series as the present facilities will allow of such rocks as have been the 

 subject of the close methods of scrutiny adopted in modern petrography. It there- 

 fore happens that certain groups, and perhaps the eruptive rocks as a whole, are 

 represented in greater profusion than their geological importance seemingly war- 

 rants. The, system of installation is, however, by no means inelastic, and wbeu these 

 other groups shall in their turn receive the attention they merit a place can readily 

 be made for them by substitution, or, better yet, by an expansion of the entire series. 



