PEEIDOTITB INTRUSIVES. 261 



Measurements gave a symmetrical extinction of 3'2 each side of the twinning 

 plane on zone _L010. I therefo.'; conckide the feldspar to be labradorite. 



Biotite. — This is the ordinary yellow to brownish kind, and is in irregu- 

 lar plates. It shows its usual characters and is included in the hornblende. 



Magnetite. — Tliis mineral occurs in crystals and grains, included in all 

 the other constitiients. 



Quartz. — A few grains of quartz were found associated with the feldspar. 

 The presence of dihexahedral liquid inclusions easily gave a clew to the 

 orientation of the grains. 



The rock composed of the above-described minerals offers a good illus- 

 tration of that gradation which is one of the fundamental laws of nature 

 and is nowhere better exemplified than in the rocks. On the one hand, 

 from its texture and from the presence of the dominant hornblende, with the 

 small quantity of quartz, this rock may perhaps be considered to be closely 

 related to the diorites. On the other hand, the presence of the pyroxene 

 and olivine seems to point toward its connection with a gabbro. 



Its geological occurrence points most satisfactorily toward its corre- 

 spondence in age and its intimate relationship to the peridotites of the dis- 

 trict. The predominance of the bisilicates indicates it to be of very basic 

 character, and for these reasons I have called it " peridotite," although I 

 have not succeeded in getting an analysis to prove its ultrabasic nature. 



RELATIONS OF PERIDOTITES TO OTHER ROCKS. 



The peridotites occur in such small quantity that general conclusions 

 concerning their relations to other rocks occurring in their vicinity are 

 scarcely warranted. However, from the fact that they are so intimately 

 associated with the gabbro — cutting it in two cases where the contact was 

 observed — and from the fact that among the peridotites themselves certain 

 phases approach in mineralogical composition certain of the gabbros (see 

 p. 254), it seems advisable to conclude that they represent ultrabasic differ- 

 entiation products of the same magma from which the gabbro types were 

 derived. The inappreciable difPerences in grain between the portion of the 

 rock nearest the contact between these basic rocks and the gabbros and 

 those farther away can be explained by supposing their intrusion tp have 

 taken place while the main mass of the gabbro retained considerable heat 

 and thus prevented their rapid cooling. 



