52 ox MESOZOIC DOLERITE AND DIABASE. 



Calcite. — This occurs in small quantities, c.y.^ in the- 

 Org'an-pipes of Mount Wellington. 



Sf't-pentinr. — There is a little yellow serpentine in the 

 olivine-dolerites, resulting from the decomposition of 

 olivine. 



Scolecite. — This zeolite is occasionally found coating the 

 joint planes and faces of the rock at the Cataract quarry, 

 Launceston, in white radial aggregates. It is a hydrous 

 aluminium and calcium silicate, which has been reported 

 from cavities and fissures of widely differing rocks, 

 basalts, dolerite, granite. It is of no particular importance 

 as a rock-forming mineral, but forms interesting specimens 

 from the mineralogist's point of view. It is apparently a 

 decomposition product originated by the access of meteoric 

 waters. 



Ground/nass. — This occurs sparsely in most varieties 

 between the angles of the felspars, and sometimes forms 

 irregular patches in the rock. In the Hobart Railway 

 Station rock it is in sufficient quantity to produce a 

 porphyritic facies. It mostly forms an imperfectly indi- 

 vidualised mass, comprising skeletal and embryonic 

 felspars, magnetite grains, &c. Its character is well dis- 

 played in Mr. Teall's figure of sections of the High Green 

 plagioclase augite dyke, Q.J., Geol. Soc, 1884, p. XIII., 

 Fig. 2. 



Some of the small felspars in the groundmass of the 

 Hobart rock, and that of Ross, give straight extinctions, and 

 may be oligoclase. The groundmass is holocrystalline 

 f elspathic, and has not a basaltic facies. , It would aj)pear to 

 indicate that the rock did not consolidate subaerially, but, 

 on the other hand, not very far below the surface. This f el- 

 spathic gi^oundmass is plentiful in some of the dolerite near 

 Bothwell. The main mass of the rock between Bothwell 

 and The Lakes, despite its general coarseness of grain, has 

 a little of it, and it is not wholly absent fi-om the coarse 

 ophitic dolerite at the dam on St. Patrick's River. It is 

 abundant in a fresh coarse dolerite near Mount Claude, 

 which is also remarkable for containing allotriomorphic 

 felspar. In some varieties the green chlorite has w^andered 

 into the groundmass, as at Mount Direction (south), where 

 the rock cotains some olivine. 



Besides the ophitic structure, we have another modifi- 

 cation found chiefly in the finer grained varieties of this 

 rock, namely, one in which an incompletely individualised 

 or otherwise indistinct groundmass exists in the interstices 

 of the small crystals of felspar, and round the granules or 



