PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 299 



Diabase.] 



Meg. The rock is brick red, imperfectly crystalline, with a few spots or specks 

 of greenish color and some quartzes. There is also occasionally visible a glittering 

 reflection, evidently from the cleavage of a feldspar. 



Mic. The reddened section is occupied almost entirely by a network of spher- 

 uliths, some of which have bipyramidal quartz at the centre. These spheruliths 

 are but rarely true spheruliths. The reddened matter, it is true, is almost always 

 arranged in a radial structure about the quartzes, but it is observable that the quartz 

 background usually embraces this reddened matter poikilitically and extinguishes 

 in conjunction with the quartz which occupies the centre or in patches which have 

 no relation to the rays of the spherulith. In other places there is evidence that a 

 feldspar form has become reddened and occupied by silica in a micropegmatitic man- 

 ner. In still other parts, which sometimes are quite large, there is a closer grouping 

 of individual quartzes similar to those which lie at the centres of the spheruliths, 

 but their surroundings are more finely granular and complex, with a liberal supply 

 still of pegmatitic quartz. The dark portions of the slide are occupied by chlorite and 

 by magnetite. One of the slides also shows epidote. 



Three sections. 



Chemical analysis. An analysis of this rock (No. 285) gave the following results: 



Si0 2 73.91 



A1 2 8 14.89 



Fe 2 3 2.27 



FeO 1.70 



CaO .27 



MgO trace 



K 2 O 2.78 



Na 2 O 2.64 



H 2 1.01 



Total, 99.47 



Aye. Cabotian. 



Remark. This rock is strictly identical with the rock forming the knob on 

 Pigeon point peninsula from which was obtained rock No. 1845. It has been dis- 

 cussed at length by Bayley, and is again referred to in Part III, in the treatment of 

 the " red rocks" of the state. (Compare, also, Part I of this volume.) N. H. w. 



No. 286. DIABASE (?) 



"Black, basaltic rock, from the narrow dike adjoining No. 285." 

 Ref. Annual Report, ix, page 68. 



Meg. A compact, fine-grained, very dark, diabasic rock containing a little 

 pyrite. 



Mic. The section shows a much altered rock. It is composed of plagioclase, 

 much altered, hornblende, magnetite, a dirty greenisjh product, and quartz. It is impos- 

 sible to determine whether augite was originally present, but it seems quite probable 



