124 Transactions. 



to weather m concentric layers. The shells of decomposed 

 rock surrounding the boulder illustrated were more than 2 ft. 

 in thickness. In some of the smaller boulders only a kernel of 

 fresh rock remains (Plate XVII, fig. 1). 



The basic and alkaline basic rocks collected have a wide 

 range, and include an interesting series of tinguaites, tinguaite- 

 porphyries, vogesites, camptonites, diabases, and rocks ap- 

 proaching monchiquites associated with theralites and gabbro- 

 diabases. The latter may possibly be deep-seated representa- 

 tives of the other rocks ; but, until their plutonic or hypabyssal 

 origin can be determined from outcrops in the field, they will 

 be classed with the dyke rocks. 



The numbers under which the rocks are described are the 

 field numbers of the specimens as collected. Only those which 

 represent the different types, or show transitional characters of 

 an interesting nature, have been described. 



The specimens collected numbered 131, from each of which 

 one or more sections were prepared, and only in exceptional 

 cases were any two rocks found to be exactly similar. They 

 grade gradually from one type to another throughout the whole 

 series. The suggested inference is that the whole series are 

 genetically the product of one alkaline magma, which has under- 

 gone a gradual differentiation during the period in which the 

 dykes were injected. 



108. Tinguaite. Megascopieally, a semi-translucent green 

 rock, with vitreous fracture, resembling pitchstone. Micro- 

 scopically, a network of segerine crystals, with occasional 

 phenocrysts of anorthoclase distributed in a groundmass of 

 anorthoclase, cancrinite, and nepheline. The flegerine is brownish- 

 green in colour, and occurs in crystals of blade-like habit, some- 

 times frayed at the ends. It gives straight extinction and 

 moderate pleochroism, dark-green for vibrations parallel to the 

 longitudinal axis, and yellow-green perpendicular to it. It also 

 occurs as needles and microlites, without any approach to 

 orientation. It is idiomorphic to all other minerals, the terminal 

 ends of the individuals sometimes penetrating the feldspar. 

 The feldspar occurs in two generations, the earlier being idio- 

 morphic. Rectangular phenocrysts are sparingly developed : 

 crystals of long blade-like habit are frequent. Although these 

 consolidated after the pyroxene, they are only occasionally 

 penetrated by it, but appear, to have pushed the segerine aside, 

 or to have developed alongside the already crystallized pyroxene. 

 A few of the broader crystals show Carlsbad twinning. The 

 groundmass of the rock is composed of cancrinite, nepheline. 

 and anorthoclase in allotriomorphic relations: the small plates 

 of anorthoclase with ragged outlines <jive nndnlatory extinction. 



