PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 233 



Diabase. Thomsonite.] 



the much-decayed, alternating trap and amygdaloid of the coast. The darker variety, 

 which forms the upper part of the little peninsula of Sugar Loaf point, is but scantily 

 amygdaloidal, and is permeated with green thalite. It is eighteen feet thick, and 

 dips, with the underlying, 12 toward the south 10 east. The lower part of the rock 

 of the point, about seven feet thick, is more loosely amygdaloidal with calcite, and is 

 confusedly mingled as if in part made up of a breccia, or of breccia and conglomerate. 

 It is probably the upper portion of a superficial lava flow, which was subsequently 

 covered by the trap which constitutes the upper portion of the " loaf." The sample 

 collected from this lower portion of the loaf is marked by pipe-like amygdaloidal 

 cavities now filled with calcite. These pipes are, as preserved, two and a half or 

 three inches in length, and doubtless ascended vertically through the rock, marking 

 the avenues of escape of gases when the rock was cooling. They are round and about 

 one-eighth to one-fourth of an inch in diameter. On the upper surface, on the area 

 of a square inch, there would be, on an average, four or five of these openings. The 

 rock is, besides, more or less permeated by saponite (thalite). This mineral also con- 

 stitutes a thin first lining in the calcite-filled pipes. Along with calcite is also more 

 or less of thomsonite ( ?) and chalcedony. 



Mic. The thin section shows an ophitic structure and the olivine considerably 

 changed to an opaque ferruginous bowlingite ( ?) around which can be seen an excess 

 of iron oxide in the form of micaceous hematite, which spreads somewhat into the 

 feldspars and gives a negative uniaxial interference figure. In these thin deposits this 

 red mineral is laminated and pleochroic. A section of the rock of the lower portion 

 of the loaf is finer grained, reddish brown by reason of the abundant dissemination 

 of iron oxide, and carries nests of the thalite mentioned under No. 162. 



Three sections. 



Aye. Manitou. N. H. w. 



No. 163A. THOMSONITE. 



From No. 163. Large nests of a coarsely lamello-fibrous, radiated, white mineral occur in No. 163. 



MCIJ. This mineral is hard, brittle and glassy, and in being extracted breaks 

 into wedge or fan-shaped triangular pieces, coincident with the divergent fibres. It 

 also has a cleavage or jointage by which it breaks easily, at irregular intervals, trans- 

 versely to the fibrous grains. In the triangular pieces there is apparent a consider- 

 able variation, suggesting the possibility of two minerals intimately intergrown. The 

 outer peripheries of the amygdaloidal masses are coarser than the central por- 

 tions, and to that the foregoing applies. Toward the centre of each mass, as the 

 fibres become almost insensibly finer, a pink tint comes on, and at last the mineral 

 is massive rather than fibrous. There is an indistinct zigzag interlocking of the 

 structure characteristic of the outer mineral into the structure exhibited by the 



