Geologically, Sri Lanka is divided into three main groups of rock types, viz. the highland group, the South-western group and the Vijayan Complex, all consisting of Precambrian crystalline rocks. The Sinharaja lies in the transition zone between the Highland group (Figure 7). The rock types found in the Khondalites of metamorphosed sediments and charnkite of the Highland group as well as the metasediments, charnokites and scapolite bearing calc-granulites of the south-western group.

Figure 7. The Sinharaja basic-zone (adapted from
Munasinghe and Dissanayake, 1981)
	1 LIMESTONE 2 MIOCENE 3 WESTERN VIJAYAN GROUP 4 HIGHLAND GROUP 5 EASTERN VIJAYAN DROUP 6 SOUTH WESTERN GROUP

                The most significant geological feature of the forest is a distinctive zone of basic rocks which are referred to as the "Sinharaja basic zone", and which consist of horneblende, pyriclasts, basic charnokites, pyroxene amphiobolites inter-banded with minor accurrences of quartzites, garnetbiotite gneisses and intermediate chrnokites. The basic chrnokites and pyroxene amphibolites indicate an igneous origin prior to metamorphism, created by a low pressure. It is thought that this basic rock formation has led to the desilication processes in the surrounding areas of Ratnapura and Deniyaya which have gem-fields of cordierite-bearing rocks.

                The basic zone also coincides with an aeromagnetic anomaly stretching from Nawalakande through Pitakele and ending at the Denuwankanda-Beverly Estate area.