KINDS OF ROCKS. ( ( 



but with less feldspar and much more mica, — therefore glistening in 

 lustre ; slaty, or very schistose, in structure, breaking into thin slabs 

 or plates ; often friable, or wearing easily. 



Mica schist often abounds in garnets, staurotide (§ 61), or tourma- 

 line (| 63). 



The variety plumbaginous, schist contains plumbago (p. 64) in its layers. Cal- 

 careous mica schist contains, disseminated through it, carbonate of lime or cal- 

 cite. Talcose mica schist contains talc as well as mica. Hornblendic mica schist 

 contains black or greenish-black crystals of hornblende. Anthophyllitic schist 

 is related to the last, but the acicular crystals are of the brown variety of horn- 

 blende called anthophyllite. Concretionary mica schist is a variety containing 

 concretions : the concretions may consist of feldspar and contain garnets ; or 

 of mica and feldspar, etc. Specular schist is a variety in which the mica is 

 replaced to a great extent by micaceous or lamellar specular iron. It may be 

 connected either with mica schist or talcose schist. 



(6.) Argillite, or Clay-Slate, Roofing-Slate. — Mica slate gra- 

 duates into slates in which there is no distinct crystallization and 

 even the mica is not apparent. This fine-grained slate-rock is 

 ordinary roofing-slate and writing-slate, and occurs of bluish, pur- 

 plish, red, and other colors, to black. 



Argillite often contains andalusite (§ 60) running through it in 

 prismatic crystals seldom smaller than a goose-quill, and one or more 

 inches long. It also occasionally contains garnet, tourmaline, staurotide, 

 and ottrelite, — the last a mica-like though brittle mineral, imbedded 

 in small scales spangling the rock, as at Billingham, Massachusetts. 

 The ottrelite has been called phyllite. The more glistening kinds 

 of slate sometimes contain mica in distinct but minute scales, or 

 have a sort of micaceous surface. 



2. Hornblendic Series. 



84. The hornblendic series commences in a granite-like species 

 (called syenite) containing quartz and feldspar along with hornblende 

 in place of mica. Hornblende is not so cleavable into leaves as 

 mica, and is brittle instead of elastic. It is also tough and heavy ; 

 and hence hornblende rocks are generally tough and heavy, the 

 specific gravity between 2.9 and 3.5. From syenite the series runs 

 down through syenitic gneiss to hornblendic schist and hornblende 

 rock ; then to rocks of very even texture and compactness, called 

 diabase and aphanite, the last like hornstone in fracture and surface. 

 The rocks above, following syenitic gneiss and some of the schists, 

 contain no quartz. 



There is another related series, in which the rock contains hy- 

 persthene or diallage instead of hornblende, and quartz is mostly 



